You can use `useAuthenticator` composable to access current signed in `user`. If no user is authenticated, it'll return `undefined`.

```html{1,5-8}
<script>
  import { Authenticator, useAuthenticator } from '@aws-amplify/ui-svelte';
  const {authenticator: a } = $derived(useAuthenticator());
</script>

<Authenticator />
{#if a.route === 'authenticated'}
  <h1>Hello { a.user?.username }!</h1>
  <button onclick={a.signOut}>Sign out</button>
{/if}
```
